The present invention relates to a magnetorheological fluid damper and more particularly, to a linear acting fluid damper for a vehicle suspension employing magnetic tuning in connection with a magnetorheological working fluid to effect desired damping levels and further including a bypass feature.
Magnetorheological fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium are well known. The flow characteristics of these fluids can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electromagnetorheological interactive properties for advantageous use in a variety of controllable coupling and damping devices, such as brakes, clutches, and dampers.
Linear acting MR dampers have been previously proposed for suspension systems, such as a vehicle suspension system and vehicle engine mounts. One example of such a MR damper discloses a conventional linear acting controllable vibration damper apparatus that includes a piston positioned in a magnetorheological fluid-filled chamber to form upper and lower chambers. The piston includes a coil assembly, a core, i.e. pole pieces, and an annular ring element positioned around the pole pieces to form an annular flow passage for permitting flow of the magnetorheological fluid between the chambers. When the piston is displaced, magnetorheological fluid is forced through the annular flow passage. When the coil is energized, a magnetic field permeates the channel and excites a transformation of the magnetorheological fluid to a state that exhibits increased damping forces as a result of an increase of apparent viscosity of the fluid.
The damping performance of a suspension damper is largely dependent on the force-velocity characteristics of the damper. In standard suspension dampers of the prior art that do not use MR fluid, the force-velocity curve typically has a steeper slope at low velocities and desirably passes through the zero point of damping force at zero velocity, thus producing a smooth transition between damper movements in compression and extension directions. Without special design considerations, however, a suspension damper using MR fluid tends to have a force-velocity curve that intersects the force axis at a value above zero from the positive velocity side and a value below zero from the negative velocity side, thus producing a jump in force between finite positive and negative values with each change in the direction of damper movement. These jumps in force tend to provide a harshness to the vehicle ride which may be felt by the vehicle occupants.
Conventional MR dampers attempt to solve the zero intersect problem by including one or more fluid bypass passages through the piston or on the outer surface thereof, in an area of weak or no magnetic flux and not open to the main, magnetic flux controlled fluid path through the piston, e.g., in the outer surface of the flux ring. The relatively unimpeded flow of MR fluid through the outer bypass passages permits the damping curves to intersect zero. However, this design also results in an undesirable steep rise in the damping curve from the zero point followed by a sharp transition into higher velocities. In addition, the steep rise may often result in the damper overshooting the desired force at the transition. The steep slope and overshooting results in undesirable discontinuities when such a damper is used in vehicle suspensions. Specifically, the use of a totally separate bypass passage impairs the ability to achieve noise control and smooth load transfer. Also, the MR fluid flowing through the outer bypass passages is not within the magnetic flux path, is not exposed to magnetic flux and therefore, does not experience an MR effect. As a result, the outer passages represent a pure loss in pressure in the system that disadvantageously reduces the maximum force achievable.
Therefore, there is a need for an MR damper capable of effectively providing a smooth and controllable transition, without a sharp break in the damper force/velocity curve, between very low damping forces near zero damper piston velocity to higher damping forces at higher damper piston velocities while maintaining desirable maximum force levels.
The present invention is aimed at providing an MR damper capable of effectively providing a smooth transition between very low damping forces near zero damper piston velocity to a higher damping forces at higher damper piston velocities without sacrificing maximum force levels.
One aspect of the present invention provides a damper, including a cylinder containing a magnetorheological fluid. A piston is slidably mounted for reciprocal movement in the cylinder. The piston includes a core and a flux ring positioned about the core, the core and flux ring defines an annular, axially directed flow gap therebetween and at least one non-magnetic portion is positioned along the flow gap in at least one of the flux ring and the core, wherein the non-magnetic portion includes at least one groove formed therein and positioned along at least a portion of the flow gap.
Other aspects of the present invention provides a coil disposed in the piston core, wherein the at least one groove can be an interrupted groove formed above and below the coil. The at least one groove can be a plurality of axial grooves arranged about one or both of the core and the flux ring. The plurality of grooves can be formed on an outer surface of the core. The plurality of grooves can be formed on an inner surface of the flux ring. The non-magnetic material can be a polymeric material. The non-magnetic portion can be disposed in at least one axial slot formed in the flux ring. The non-magnetic portion can be disposed in at least one axial slot formed in the core. The at least one groove can extend along an entire length of the flux ring in communication with the flow gap along an entire length of the groove.
Another aspect of the present invention provides a method for providing a smooth transition between low and high velocity damping forces in a fluid damper for a vehicle suspension including generating a predetermined flux level through a gap formed between a first and second chamber of the damper, wherein the gap comprises an annular, axially directed passage within a piston assembly of the damper and generating a decreased flux level through at least one groove, an entire length of the groove formed in communication with the gap, wherein the groove is formed in a non-magnetic portion of the piston and axially directed along at least a portion of a length of one of an inner wall and an outer wall defining the annular, axially directed passage.
Another aspect of the present invention provides a damper for providing a smooth transition between low and high velocity damping forces in a fluid damper for a vehicle suspension including a means for generating a predetermined flux level through a gap formed between a first and second chamber of the damper, wherein the gap comprises an annular, axially directed passage within a piston assembly of the damper and a means for generating a decreased flux level through at least one groove, an entire length of the groove formed in communication with the gap, wherein the groove is formed in a non-magnetic portion of the piston and axially directed along at least a portion of a length of one of an inner wall and an outer wall defining the annular, axially directed passage.
Another aspect of the present invention provides a damper, including a cylinder containing a magnetorheological fluid and a piston assembly slidably mounted for reciprocal movement in the cylinder to form a first chamber positioned on one side of the piston assembly and a second chamber positioned on an opposite side of the piston assembly. The piston assembly includes a fluid flow passage therethrough defining a flow gap. The flow gap permits limited fluid flow between the first chamber and the second chamber and includes a groove adjacent and open to the flow gap along at least a portion of the flow gap in the direction of fluid flow. The groove is formed in a portion of the piston assembly comprised of a non-magnetic material. The groove is axially directed along one of an inner wall and an outer wall defining the flow gap. The piston assembly further includes a magnet assembly establishing a magnetic flux circuit through the flow gap and through the groove in series with a region of the flow gap adjacent the groove. The magnetic flux is established through the flow gap and groove transversely to the direction of fluid flow therethrough to cause magnetorheological fluid flowing through the flow gap and the groove to experience a magnetorheological effect affecting the flow of the magnetorheological fluid therethrough, whereby the groove and non magnetic portion of the piston causes the magnetic flux density therethrough to be smoothly reduced from that in the remainder of the flow gap.
Other aspects of the present invention provides a damper wherein the magnetic flux circuit including magnetic poles defining the flow gap, the flow gap and groove in series providing a greater gap width thereacross between the magnetic poles than the flow gap alone. The groove can be formed by a curvilinear surface. The curvilinear surface can be a circular arc. The groove can be formed by a triangular surface. The groove can be largely rectangular or trapezoidal in cross-section. The shape and size of the groove can be tuned to provide the desired force vs. velocity curve. The annular, axially directed passage could include a plurality of grooves formed therealong. The groove can be lined with a non-magnetic material adapted to lower the flux density in the groove.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.